Duplex aluminous products and articles



April 24, 1956 NQCK JR DUPLEX ALUMINOUS PRQDUCTS AND ARTICLES Filed June 18, 1952 INVENTOR. Jose /z A. Neck, -75. BY

n States Patent ApplicationiJune. 18,1952, Serial No. 294,194 SCiaim's. ((31. 29-1975) EI he invention relates in general to the provision of duplex aluminous eprodu'cts and articles in the form of high :strength aluminum basealloys provided with an adherent vprotective coating of aluminum base alloy on one or more surfaces thereof. -More specifically the duplex aluminous products contemplated within the scope --of the invention are those which comprise an aluminous base or core member of aluminum zinc magnesiumcopperralloy, protectively coated with an aluminum alloy containing zinc as' the essential added component.

It is-well known that certain commercial aluminumzinc-magn-esium alloys, in. the solution heat treated and artificially aged condition, exhibit desirable high strength values. It is also generally known that additions of copper in small amounts, up to and including 1.6 per cent by weight, has resulted .in increasing the strength of these commercial aluminum-zinc magnesium alloys. Examples of wrought alloys in the solution :heat treated and artificially aged condition, which have received commercial acceptance in this general category. are listed herein in Table I in terms of their nominal commercial composition and typical mechanical properties, as follows:

Table -I Percentage Alloy Comp osi ti on B a] an e e Mechanical Properties Aluminum Example j I Teusile Yield g QQ Cu Zn M Mn, Strength Strength lb./sq.iu. lb./sq.in. c m

Regardless of the desirable strength characteristics of thisgeneral class of. alloys, it' has been found that larger amounts of copper have a pronounced tendency to reduce ttheircorrosion resistance, particularly in the low magnesium and high zinc ranges. Furthermore, the presence of nickel, even in relatively small amounts,--has a tendency to increase the susceptibility to corrosion of this :general type of alloy composition. Since these alloys have otherwise. desirable properties, especially :high strength, their reduced resistance to'corrosion has been ahandicap. The application' of a high purity aluminum coating in the manner employed with aluminum-copper type alloys has not;provided adequate protection. It has, therefore, been a problem before 'the art to satisfactorily protect aluminum-zinc-magnesium alloys against corrosion in their normal use within a magnesium-zinc range that otherwise-makes these alloys desirable from -,a strength standpoint. i

The art-is also cognizantof the fact that the addition of cert'ain'elements to commercially pure aluminum, as disclosed in United States .Letters Patent 1,997,165, granted/April .9, ,1 935, to Robert H. Brown, provides 2,742,688 Patented *A-pr. :24, .1956

coating alloys having higher solution potentialthanpure aluminum and many aluminum :base alloys and, therefore, provides better protective coatings.

:I have discovered, :however, that with the aforesaid knowledge available to the art, the application of protective coatings of aluminum base alloys containing -2i'nc, as the essential added component, oncertain'aluminumzinc-magnesium alloy cores containing .small amounts of copper does not in all instances provide a duplex aluminous product having satisfactory resistance to'corro sionin'the solution h'ea't'treated and "artificially. aged conditi'on. For example, certain duplex aluminous products made in accordance with prior practice exhibit surface blistering and severesub-surfacecorrosive attack, particularly along their exposed edges, below the externalsurface and in the vicinity of a zone or layer between-the base and coating alloy. This type of corrosion is defined herein as -moletrack or undermining, and results from a higher solution potential-exhibited by theaforementioned intermediate zones or layers as compared to the base and coating alloys. The intermediate zone or layers .giving rise to :the mole-track type corrosion. are presumably formed by diifusion of the .zinc and magnesium elements of the base material into :the coating alloys, which gives rise to local formation of an interface alloy creatingza condition of concentration gradients which exhibit relatively high solution potentials. This condition is particularly noticeable when the core alloys contain zinc. in. amounts of "6 per cent and higher.

It is=the primary object of the present invention to providea solution heat treated and artificially aged: aluminous product or article composed of relatively high strength aluminum-zinc-magnesium-copperalloy, substantially free from nickel, with higher amounts :of 'copper than heretofore contemplated commercially, and 6 per cent or more zinc, and provided with an adherent aluminum alloy protective coating, which duplex product or article is characterized by freedom from zones or layers of :metal-lic constituents intermediate the base and the coming that exhibit solutionpotentials higher than those of the base and coating-alloys.

It is another object of the invention to provide an aluminous product 'or article, in the solution heat treated and artificially aged condition, the core or base being composed of relatively high strength aIuminurn-Zinm magnesium-copper alloy, substantially free from nickel, with higher amounts of copper than-heretofore conten'n plated commercially, and -6 or more per cent zinc, and provided with an adherent aluminum alloy protective coating, which product or article is characterized by freedom from mole-track corrosive attack .in corrosive environments. 1

Other objects and advantages will be understood by those skilled in this "art on consideration of vthe following specification and drawing, in which mole-"track corrosion has been schematicallyillustrated.

The duplex aluminous products orv articles of the :iu vention are provided on one or=nuire surfaces with the protective coating alloy. In forming the duplex product any known methodmay be employed to achieve adherent bond between the "core and protective coating. for-example, the protective coating may be applied by rue-- chanical working of a superimposed Wrought-sheet on .a cast or wrought base or core, as by rolling tortother mechanical Working step. The adherent protective coating may also be obtained by' metal spraying and/orcast claims, is 'rireaut anyiheat treatmenti'which'compri'sessheet ing the duplex products or articles of the invention to a temperature and for a time sufiicient to cause solution of a substantial quantity of the soluble elements of the alloys, but below the fusion temperature of the lowest melting point constituent. For example, heating the duplex aluminous products or articles between about 800 F. and 975 F. for a period from about five minutes to five hours, followed by quenching to room temperature, is typical for the products of the invention.

'The solution heat treated duplex articles or products of the invention must be artificially aged. By artificial aging, as used herein, is meant any treatment that serves to improve the hardness and strength of the solution heat treated duplex aluminous products, as for example, heating to temperatures between about 200 F. to 350 F. for about two to fifty hours and thereafter allowing the prod ucts to cool to room temperature. The artificial aging may be carried out at a constant selected temperaure for the entire period of aging, or it may be carried out at progressively increasing temperature, within the temperature range stated above, in either continuous or stepped fashion. Furthermore, the aging schedule may be divided into predetermined periods of heating with cooling to room temperature between said periods.

': I have discovered that solution heat treated and artificially aged duplex aluminous products, characterized by substantial freedom from mole-track corrosive attack, can be produced without loss in strength by providing in the aluminum base alloy core member a copper content of about 1.9 to 4.5 per cent, while limiting the magnesium to between about 1 and 3.5 per cent and the zinc between about 6 and 9 per cent, the ratio of the copper to zinc contents being maintained between .2 to .7, balance principally aluminum, and still use the aluminum base alloy coating named above, in which zinc is the essential added component between the limits of about 0.5 and 2.5 per cent, balance principally aluminum.

- By aluminum base alloy," in reference to the core and being present in amounts exceeding the zinc present in the coating alloys. Copper may also be present in the coating alloys although this element has a tendency to lower the solution potential of the protective coating below a desired minimum, which will be explained in more detail hereinafter. For this reason, copper, if present in the coating alloys, should never exceed about 0.05 per cent.

By controlling the copper to zinc ratio in the base or core alloys of the type described above, I have found that the core alloys will exhibit a solution potential in the heat treated and artificially aged condition, of not more than 0.840 volt. The protective coatings, on the other hand, exhibit solution potentials of not less than 0.925 volt. A calomel electrode was used as a standard against which the potentials of the base and coating alloys referred to above, and hereinafter, were measured, the electrolyte used being a normal solution of sodium chloride containing 0.3 per cent by weight of hydrogen peroxide.

The drawing attached hereto and forming a part hereof is provided for purposes of schematically illustrating moletrack corrosion and its accompanying blistering attack, as observed in actual practice. The drawing represents a corroded section of 2. rolled sheet of duplex aluminous material, in the heat treated and artificially aged condition, comprising an aluminum-zinc-magnesium-copper alloy core 10 provided on both major surfaces with an aluminum-zinc coating alloy 12, in which the solution potential of the core is higher than the permissible maximum 0.840 volt, and the copper to zinc ratio is below .2. Exaggerated mole-track undermining and accompanying superficial blistering are represented by reference numerals 14 and 15, respectively.

Typical examples of duplex products responding to the teaching and practice of the invention are listed in Table II under explanatory column headings, the right hand column in each instance representing the difference in solution potential (volt) between the core and protective coating.

Table II Core or Base Percentage Gompositlon Solution Coating Percentage Composi- Solution Difierence sheet Balance Aluminum Culzn Poterfttml tion-Balance Aluminum Potelfltial S linl o 0 out on sample Rat) Core Coating Potential Cu Zn Mg Si Fe Mn Cr Ti Be (volt) Zn Cd 811 Si Fe Cn (volt) (volt) coating alloys, is meant alloys containing at least 70 per cent aluminum. The term balance principally aluminum," in reference to the core and coating alloys is meant to include possible additions of various elements, as well as the usual impurities iron and silicon normally present in commercial aluminum, any or all of which have no effect on the end products of the invention.

For example, the core alloys may include possible additions of at least one of the hardening elements chromium and manganese in total amount up to about 2 per cent by weight, and/ or at least one of the grain refining elements boron, columbinm, molybdenum, tantalum, titanium, tungsten, vanadium and zirconium in total amount up to about 1 per cent by weight. Beryllium in amounts between about 0.0005 and 0.05 per cent and lithium plus beryllium in amounts between about 0.001 and 0.02 per cent, when added to the core or base alloys, prevent buming losses in the molten state, but otherwise have no affect on the end products of the invention.

In respect to the coating alloys, possible additions of at least one of the elements tin and cadmium may be included providing the tin is kept between about 0.01 and 0.5: per cent and the cadmium is kept between about 0.01 and 1.5 per cent, the tin and/or cadmium never All of the duplex aluminous products listed in Table II were produced in the form of rolled sheet 0.064 inch thick provided on each surface with a protective coating controlled at about four per cent of the total thickness of the duplex product in each instance. Furthermore, all of the duplex products were solution heat treated and artificially aged to develop their strength and hardness properties.

In the case of sheet sample I, the duplex aluminous product was heat treated fifteen minutes at 920 F cold water quenched and artificially aged twelve hours at 275" F. Sheet sample 11 was heat treated twenty minutes at 880 F., cold water quenched and artificially aged four hours at 210 F., followed by eight hours at 315' F. Sheet samples III through VIII were heat treated twenty minutes at 860 F., cold water quenched and artifically aged four hours at 210 F., followed by eight hours at 315 F.

It will be observed that all of the aluminum base alloy core materials listed in Table 11 fall within the composition range 1.9 to 4.5 per cent copper, 6 to 9 per cent zinc, and 1 to 3.5 per cent magnesium, and exhibit solu-' tion potentials in the solution heat treated and artificially aged condition of not greater than 0.840 volt, and have.

arm ess a ratio of copper to zinc between .2 to ;7; It should also be noted" that the alloys listedjincluded 'thehardeners and one of the grain refiners hereinbe'fore stated, as well as small amounts of beryllium (sheet"s'ample H), and that the core alloys are substantially free from nickel, i. e., lessthan 0. 10 per cent nickel calculated on a weight basis. i

"Referring inmore detail "to TableII, *ten'sile specimens cut from the duplex aluminous sheet sample I were exposed in stressed (75 per cent of yield strength) and unstressed condition 'to an alternate immersion 'test in a standard 5 per cent sodium chloride-03pm cent hydrogen peroxide aqueous 'soluti'onfor aperiod of one .week.

es'pecimens were examined and their tensilejproperties determined following their exposure. Examination of the specimens revealed solution o fjthe coating alloy to a depth slightly above the 'difiusion zone, complete absence of stress corrosion cracking, and complete absence of mole-track corrosion and its .normalaccompanying surface "blistering, all of which is indicative ofcomplete c'ore protection. Mechanical test results'reveale'd negligible loss initensile properties. Solution potential measuremerits for the coating and core alloysjwere 0.950 and :838 volt, respectively. v Tensile specimens cut from the-duplexaluminous sheet sample II were subjected to the following corrosion treatments:

1') "Stressed (75' percent of yield strength) and unstressed specimens were exposed to ninety-six hours immersion in boiling '6 per cent sodium chloride aqueous solution. 1

(2) Stressed (75 per cent of yieldystrengthiland unstressed specimens were exposed one week to an alternate immersion test in a standard 5 ,per centsodium chloride-0.3 per cent hydrogen peroxide aqueous 'solution.

(.3) Stressed (75 per cent of yield strength) and unstressed specimens were exposed twelve we'eks to an alternate immersion test in a 3.5 percent sea salt aqueous solution, as well as nine months exposure to alternate immersion'in the same solution.

(4) A stressed preformed specimen was exposed to ninety-six hours immersion in a standard 5 per cent sodium chloride-0.3 per cent hydrogen peroxide aqueous solution.

The specimens of sheet sample II were all examined and tensile properties determined in all cases, with the exception of the preformed specimen, following exposure. The examination revealed complete absence of stress corrosion cracking and complete absence of mole-track corrosion and surface blistering. Mechanical test results revealed negligible loss in tensile properties. Solution potential measurements for the coating and core alloys were 0.950 and 0.8 volt, respectively.

Tensile specimens cut from duplex aluminous sheet sample IV were exposed in stressed (75 per cent of yield strength) and unstressed condition to an alternate immersion test in a standard 5 per cent sodium chloride-0.3 per cent hydrogen peroxide aqueous solution for a period of one week. Similar specimens were also exposed in the same stressed and unstressed condition to an alternate immersion test in a 3.5 per cent sodium chloride aqueous solution for a period of one year. Sheet panels cut from sheet. sample IV were also exposed to one year alternate immersion in a 3.5 per cent sodium chloride aqueous solution. The tensile specimens and sheet panels were examined and the tensile properties of the former were determined following their exposure. The examination revealed complete absence of mole-track corrosion and surface blistering, as well as complete absence of any stress corrosion cracking. Mechanical test results re vealed negligible loss in tensile properties. Solution potential measurements for the coating and core alloys were 0.926 and 0.819 volt, respectively.

Tensile specimens cut from sheet samples III, V, VI,

VII and VIII were exposed in stressed "(75 per "cent yield strength) and unstressed condition to alternate immersion tests, as follows: p p (1') One week in a standard "Sper cent sodium cine ride-0.3 per cent hydrogen peroxide aqueous solution, and

' :(2) One year in a 3.5 sodium chlorideaqueous solution. vAll of the'exposed specimens were examined and their tensile properties determined following exposure. Examination revealed complete absence of mole-track corros'ion and surface blistering, as well as. complete absence of/stress corrosion cracking. "No appreciable losses in tensile properties were observed for any of the specimens. Solution potential measurements forthe core and coating'alloys were:

' Y Core Solution Coating Solution sample Potential (volt) remnant-volt) III. 0. 819 0.925 0. 810 0.000

Six alloy compositions, together withtheir solution potentials in the heat'treated and artificially aged' condition, are listed in Table III following.

Table "III Percentage Composition Balance Solu- Aluminum tion Example Ratio, Patena Cu Zn Mg 'Si Fe Mn Cr Il (volt) These additional alloy compositions are further exam ples of high strength core or base alloys which, when coated with a protective aluminum alloy layer containing zinc as the essential added component within the range 0.5 to 2.5 per cent, fulfill the requirements of the invention.

It is to be understood that the specific examples appearing in the specification have been selected for the purpose of illustrating the invention and are not to be taken as limiting its scope. It is also intended that known equivalents can be employed without departing from the invention, as defined in the appended claims:

What is claimed is:

l. A duplex aluminous product in the solution heat treated and artificially aged condition composed of a substantially nickel-free aluminum base alloy core member containing as the essential added elements from about 1.9 to 4.5 per cent copper, from about 6 to 9 per cent zinc, the ratio of copper to zinc being within the range .2 to .7, and from about 1 to 3.5 per cent magnesium, balance principally aluminum, said core member exhibiting a maximum solution potential of 0.840 volt, a protective aluminum base alloy coating on said core member, said coating alloy containing zinc as the essential added component within the range 0.5 to 2.5 per cent, balance principally aluminum, said coating alloy exhibiting a minimum solution potential of 0.925 volt, and said duplex aluminous product being characterized by subst'antial freedom from mole-track corrosion.

2. A duplex aluminous product in the solution heat treated and artificially aged condition composed of a sub- 2,742,ess

principally aluminum, said core member exhibiting a maximum solution potential of 0.840 volt, a protective aluminum base alloy coating on said core member, said coating alloy containing zinc as the essential added element between about 0.5 to 2.5 per cent, balance principally aluminum, said coating alloy exhibiting a minimum solution potential of 0.925 volt, and said duplex aluminous product being characterized by substantial freedom from mole-track corrosion.

3. A duplex aluminous product in the solution heat treated and artificially aged condition composed of a substantially nickel-free aluminum base alloy core member containing as the essential added elements from about 1.9 to 4.5 per cent copper, from about 6 to 9 per cent zinc, the ratio of copper to zinc being within the range .2 to .7, and from about 1 to 3.5 per cent magnesium, balance principally aluminum, said core member exhibiting a maximum solution potential of 0.840 volt, a pro- 0.925 volt, and said duplex aluminous product being characterized by substantial freedom from mole-track corrosion.

4. A duplex aluminous product in the solution heat treated and artificially aged condition composed of a substantially nickel-free aluminum base alloy core member containing as the cssential added elements from about 1.9 to 4.5 per cent copper, from about 6 to 9 per cent zinc, the ratio of copper to zinc being within the range .2 to .7,-and from about 1 to 3.5 per cent magnesium, with up to 1 per cent in total amount of at least one of the grain refining elements selected from the group consisting of boron, columbium, molybdenum, tantalum, titanium,

tungsten, vanadium and zirconium, balance principally aluminum, said core member exhibiting a maximum solution potential of 0.840 volt, a protective aluminum base alloy coating on said core member, said coating alloy containing zinc as the essential added component between about 0.5 to 2.5 per cent, balance principally aluminum, said coating alloy exhibiting a minimum solution potential of 0.925 volt, and said duplex aluminous product being characterized by substantial freedom from mole-track corrosion.

5. A duplex aluminous product in the solutionheat treated and artificially agedcondition composed of a substantially nickel-free aluminum base alloy core member containing as the essential added elements from about 1.9 to 4.5 per cent copper, from about 6 to 9 per cent zinc, the ratio of copper to zinc being within the range .2 to .7, and from about 1 to 3.5 per cent magnesium, with up to 2 per cent in total amount of at least one of the hardening elements selected from the group consisting ofchromium and manganese, balance principally aluminum, said core member exhibiting a maximum solution potential of 0.840 volt, a protective aluminum base alloy coating on said core member said coating alloy containing zinc as the essential added component in amounts between 0.5 to 2.5 per cent, balance principally aluminum, said coating alloy exhibiting a minimum solution potential of 0.925 volt, and said duplex aluminous product being characterized by substantial freedom from mole-track corrosion.

References Cited in the file of this-patent UNITED STATES PATENTS 1,418,303 Hall June 6, 1922 1,927,945 Meissner Sept. 26, 1933 1,997,165 Brown Apr. 9, 1935 2,026,574 Kempf Jan. 7, 1936 2,208,186 Tgarashi July 16, 1940 2,290,016 Bonsack July 19, 1942 2,301,759 Stroup Nov. 10, 1942 2,354,006 Gauthier July 18, 1944 2,376,681 Gauthier May 22, 1945 2,403,037 Zelgler July 2, 1946 

1. A DUPLEX ALUMINOUS PRODUCT IN THE SOLUTION HEAT TREATED AND ARTIFICIALLY AGED CONDITION COMPOSED OF A SUBSTANTIALLY NICKEL-FREE ALUMINUM BASE ALLOY CORE MEMBER CONTAINING AS THE ESSENTIAL ADDED ELEMENTS FROM ABOUT 1.9 TO 4.5 PER CENT COPPER, FROM ABOUT 6 TO 9 PER CENT ZINC, THE RATIO OF COPPER TO ZINC BEING WITHIN THE RANGE .2 TO .7, AND FROM ABOUT 1 TO 3.5 PER CENT MAGNESIUM, BALANCE PRINCIPALLY ALUMINUM, SAID CORE MEMBER EXHIBITING A MAXIMUM SOLUTION POTENTIAL OF 0.840 VOLT, A PROTECTIVE ALUMINUM BASE ALLOY COATING ON SAID CORE MEMBER, SAID COATING ALLOY CONTAINING ZINC AS THE ESSENTIAL ADDED COMPONENT WITHIN THE RANGE 0.5 TO 2.5 PER CENT, BALANCE PRINCIPALLY ALUMINUM, SAID COATING ALLOY EXHIBITING A MINIMUM SOLUTION POTENTIAL OF 0.925 VOLT, AND SAID DUPLEX ALUMINOUS PRODUCT BEING CHARACTERIZED BY SUBSTANTIAL FREEDOM FROM MOLE-TRACK CORROSION. 